1. Field of the Invention
The present invention relates to an air-fuel ratio controller for an internal combustion engine, and more particularly to an internal combustion engine air-fuel ratio controller for exercising feedback control over the air-fuel ratio in accordance with the output signal of an oxygen sensor that is installed downstream of a catalyst, and learning a steady component contained in a feedback control signal.
2. Background Art
A conventional controller disclosed, for instance, by Japanese Patent Laid-Open No. Hei7-197837 and Japanese Patent Laid-Open No. 2004-183585 controls the air-fuel ratio in accordance with the output signal of an A/F sensor, which is installed in an exhaust path and positioned upstream of a catalyst (three-way catalyst), and with the output signal of an O2 sensor, which is installed in the exhaust path and positioned downstream of the catalyst. The A/F sensor is an oxygen sensor that exhibits a linear output characteristic in relation to the air-fuel ratio. The O2 sensor is an oxygen sensor having such an output characteristic that its output suddenly changes on the rich side and lean side with reference to the theoretical air-fuel ratio. In an air-fuel ratio controller having these two oxygen sensors (hereinafter referred to as the conventional controller), the fuel amount is feedback-controlled in accordance with the output signal of the A/F sensor so that the air-fuel ratio of an exhaust gas flowing into the catalyst coincides with a target air-fuel ratio (this control operation is hereinafter referred to as main feedback control). In addition to this main feedback control, another control operation is also performed to correct the output signal of the A/F sensor in accordance with the output signal of the O2 sensor (this control operation is hereinafter referred to as sub feedback control).
In main feedback control, the conventional controller calculates a feedback control signal from the deviation between the output signal of the A/F sensor and a target signal based on the target air-fuel ratio. The target air-fuel ratio, which is used for main feedback control, is set to an air-fuel ratio (usually a theoretical air-fuel ratio) that allows the catalyst to purify the exhaust gas with the highest efficiency. However, the actual air-fuel ratio of the exhaust gas may deviate toward the rich side or lean side and away from the theoretical air-fuel ratio due to A/F sensor zero output point displacement, output characteristic changes, and other factors although main feedback control is exercised. The catalyst is capable of occluding oxygen. It maintains the catalyst atmosphere at a level close to the theoretical air-fuel ratio by occluding/discharging oxygen. However, if the exhaust air-fuel ratio continuously tends to deviate toward the rich side, the oxygen occluded by the catalyst is depleted so that HC and CO, which are contained in the exhaust gas, cannot be purified. If, on the other hand, the exhaust air-fuel ratio continuously tends to deviate toward the lean side, the oxygen occluded by the catalyst reaches saturation so that NOx cannot be purified.
Sub feedback control is exercised to complement a main feedback control operation and improve the emission characteristic of an internal combustion engine. In sub feedback control, the conventional controller calculates the correction amount for the A/F sensor output from the deviation between the output signal of the O2 sensor and a reference signal based on the theoretical air-fuel ratio, and corrects the output signal of the A/F sensor accordingly. This ensures that the deviation of the exhaust air-fuel ratio from the theoretical air-fuel ratio is reflected in the feedback control signal for main feedback control. It is therefore possible to exercise accurate air-fuel ratio control by compensating for air-fuel ratio control error that is caused, for instance, by A/F sensor zero output point displacement.
In sub feedback control, however, a steady component contained in a sub feedback control signal is also learned as a feedback learning value (sub feedback learning value). When the sub feedback learning value is added to the output signal of the A/F sensor, the air-fuel ratio is corrected so as to compensate for the above error. This ensures that the actual air-fuel ratio can be rendered close to the theoretical air-fuel ratio immediately after the start of sub feedback control.
The sub feedback learning value can be continuously learned while sub feedback control is exercised. However, if, for instance, the amount of fuel injection is cut for deceleration or increased for acceleration during sub feedback control, the catalyst atmosphere considerably deviates toward the lean side or rich side and away from the vicinity of the theoretical air-fuel ratio. If learning is conducted under such conditions, the sub feedback learning value becomes unstable, thereby increasing the deviation between the reference signal and the output signal of the O2 sensor.
To prevent the sub feedback learning value from becoming unstable, it is preferred that learning be completed at a certain point of time to fix the sub feedback learning value without allowing learning to be conducted continuously. To provide an excellent emission characteristic in such a case, it is demanded that learning be completed when a sub feedback learning value for making full use of the catalyst's purification capability is obtained.